This invention relates generally to residential furnaces and, more particularly, to a pneumatic approach for protecting against Flame rollout and blocked vent conditions.
Pressure drop sensing systems have been used in residential furnace heat exchangers since the early 1980s in order to verify or control the amount of excess air. The systems were based on the relationship of the heat exchangers pressure drop being proportional to excess air, with the system being prevented from operation if there is insufficient excess air. In variable speed inducer systems, the speed of the inducer motor is controlled in response to that pressure drop in order to maintain the desired level of excess air. Examples of such systems are described in U.S. Pat. Nos. 4,703,747 and 4,729,207, assigned to the assignee of the present invention. Such systems use one or more pressure switches to accomplish that function.
There are certain abnormal conditions that may occur in residential furnaces which cause undesirable high temperatures to exist in localized areas. One of those conditions is a flame rollout, which may occur at any location around the burners. Flame rollout is brought about by insufficient combustion air flow. The common approach for detecting a flame rollout condition is to provide one or more bimetal or fusible link switches in those positions in which of a flame rollout may occur, with the tripped switch then causing the furnace to shut down. With this approach, more than one switch may be required in order to provide the desired protection. This is particularly true in the case of a multipoise furnace, wherein the furnace (and the burners) may be placed in any one of four positions, thereby increasing the number of locations in which such a flame rollout is likely to occur. It can therefore be relatively expensive to equip the system with the required number of bimetal switches in order to obtain the desired protection.
Another undesirable condition that may occur to cause high temperatures is that of a blocked vent. If for some reason the flow of exhaust gases through the furnace vent is blocked, the temperatures in the vent area will rise to a higher than normal level. A common approach for addressing this condition is to provide a temperature switch in the area involved so as to sense a high-temperature condition and shut down the furnace. Typical of such an approach is shown in U.S. Pat. No. 4,401,425 assigned to the assignee of the present invention. While a single switch will normally meet the needs for this function, a multipoise furnace with its various possible orientations, can complicate the problem of proper location selection for the temperature sensing switch. In some applications, the temperature switch has been replaced with a pressure switch that senses the vent or inducer pressure to determine when there is a blocked vent condition to responsively shut down the furnace.
It is therefore an object of the present invention to provide an alternative method and apparatus for detecting a flame rollout condition in a furnace.
Another object of the present invention is to provide an alternative method and apparatus for detecting a blocked vent condition in the furnace, where temperature sensing means are used to detect a blocked vent.
Yet another object of the present invention is that of reducing the cost of providing protection against flame rollout conditions in a furnace.
These objects and other features of advantages become readily apparent upon reference to the following description when taken in conjunction with the appended drawings.
Briefly, in accordance with one aspect of the invention, the temperature sensing switch(es) is replaced with a pneumatic fusible element connected to the existing pressure sensing system in the furnace. The fusible element is strategically placed adjacent to the burners so as to be exposed to the high temperature in the event of a flame rollout or over temperature condition. The fusible element is composed of a material which melts and breaches at a predetermined temperature related to the flame rollout condition, such that, upon melting, the pressure therein goes to ambient to thereby shut down the furnace.
In accordance with another aspect of the invention, a pneumatic fusible element is strategically placed in the area in which the temperature would rise in the event of a blocked vent, and the fusible element is connected into the existing pressure sensing system of the furnace. The fusible element is composed of a material which melts and breaches at a predetermined temperature related to the higher temperature that would occur in the event of a blocked vent, such that, upon melting, the pressure therein goes to ambient to thereby shut down the furnace.
In the drawings as hereinafter described, a preferred embodiment is depicted; however, various other modifications and alternate constructions can be made thereto without departing from the true spirit and scope of the invention.